1. Field of the Invention
The present invention relates generally to a thermal insulation system and method of insulating in non-vacuum applications.
2. Description of Related Art
It is often necessary or desirable to limit heat transfer from an object to its surroundings. Heat transfer is the transfer of energy resulting from a temperature differential between the object and its surroundings. Heat transfer occurs through four fundamental mechanisms: radiation, solid conduction, gas conduction, and convection. The total heat transfer through any thermal insulation system will always be a combination of these four mechanisms. The dominant mechanism(s) will depend on the operational environment and the level of heat transfer.
Conduction generally involves the transfer of energy of motion between adjacent molecules, such as vibration of atoms in a crystal lattice or random motion of molecules in a gas. As such, conduction requires physical contact to affect heat transfer. The thermal conductivity, or k, is a function of the molecular state of the conducting medium. Accordingly, it is generally considered to be dependent upon temperature and pressure. Lower values of k result in a reduction in heat transfer. Heat transfer or movement of thermal energy occurs in the direction of decreasing temperature.
Convection involves the transfer of heat due to bulk transport and mixing of macroscopic elements of a fluid. Convection is thus more complicated than conduction as fluid dynamics play a significant factor in the rate of heat transfer. The heat-transfer coefficient, h, is a function of the properties of the fluid, the geometry and surface characteristics of the object surface, and the flow pattern of the fluid. Convection can by induced by density differences within the fluid medium, i.e., natural convection, or motion may be the result of external effects, i.e., forced convection. Because convective heat transfer relies on transport within a fluid medium, this component usually becomes a minor component of the total heat transfer at pressures below about 10 torr or a major component at pressures above about 100 torr.
Radiation is the transfer of heat by electromagnetic radiation, or photons. Radiation transfer is dependent upon the absorptivity, emissivity and reflectivity of the body radiating energy, i.e., the source, and the body at which the radiation impinges, i.e., the sink. There is a strong dependence of the heat-transfer coefficient on temperature as an object's radiation, and thus the heat transfer medium, will depend largely on its temperature. Although radiation transfer may occur through gases, liquids or solids, these media will absorb or reflect some or all of the energy. Accordingly, radiation transfer occurs most efficiently through an empty, vacuous space.
One common thermal insulation used in cryogenic and aerospace applications is known as Multilayer Insulation (MLI), or Superinsulation. The development of MLI around 1960 was spurred on by the space program and generally contains multiple layers of reflective material separated by spacers having low conductivity. MLI systems are intended for use in evacuated (vacuum) environments.
Ideal MLI consists of many radiation shields stacked in parallel as close as possible without touching. Low thermal conductivity spacers are employed between the layers to keep the highly conductive shields from touching one another. MLI will typically contain on the order of 50 layers per inch. MLI is thus anisotropic by nature, making it difficult to apply to complex geometries. MLI is generally very sensitive to mechanical compression and edge effects, requiring careful attention to details during all phases of installation. Accordingly, performance in practice, even under laboratory conditions, is often several times worse than ideal.
In addition, MLI is designed to work under high vacuum levels, i.e., below about 1×10−4 torr. Not only does this require lengthy evacuation, purging and heating cycles to obtain such high vacuum levels for proper performance, but such systems require either dedicated pumping systems or adsorbents and chemical gettering packs to maintain their high vacuum. Furthermore, performance of MLI degrades rapidly upon loss of such high vacuum levels. For example, a slight change from 1×10−5 torr to 1×10−3 torr can double the heat transfer through the MLI system.
Layered Composite Insulation (LCI) systems for high vacuum or soft vacuum have also been developed. However, this technology is primarily targeted to vacuum type systems where soft vacuum (1-10 torr) systems for intermediate performance is defined or high van systems where back-up performance for the system that depends on lower vacuum is designed. Such LCI systems generally require a sealed outer envelope (i.e., vacuum jacket) for both the creation of a vacuum annular space and the protection of the materials from the environmental elements.
Another common insulation is foam insulation. Foam insulation is generally intended for ambient pressure (no vacuum) applications. Foams generally have reduced thermal conductivity given their small cell sizes and relatively low densities. Furthermore, foams inhibit convective heat transfer by limiting convection to the individual cells, fissures, or other spaces within the foam structure for sub-ambient temperature applications. Foam insulation materials for sub-ambient temperature applications are predominately closed cell (but could be about half open cell for some materials such as polyimide foams) and often include some form of vapor barrier as moisture accumulation within the spaces of the foam structure can rapidly increase the thermal conductivity through the foam system. Typical foam systems include polyurethane foam, polystyrene, polyimide foam, and cellular glass (FOAMGLAS®). Conventional non-vacuum systems for piping, for example, are cellular glass, rigid foam (polyurethane or polystyrene), or spray-on polyurethane foam. The first two require the difficult and expensive sealing of seams and butt-joints and are impractical to effectively insulate flanges, pipe supports, valves, and other obstacles. Sealing is imperfect and prone to deterioration over a short time. The spray foam is often not an option for many systems because of the complexity of the component and the foaming over of all components which prevents maintenance and adjustments to the system at large. In all three categories the common feature is environmental degradation, thermal cycle cracking, water entrapment, thermal conductivity increase, corrosion under insulation, and very costly downtimes for stripping and replacement. Added to these problems are the fragile nature of the materials which tend to get compromised or even destroyed by natural engineering mechanical loadings and normal work activities.
Foam insulation is widely used in cryogenic and other sub-ambient applications but is subject to the limitations mentioned above. Such insulation is prone to cracking due to thermal cycling and cellular degradation due to environmental exposure. Cellular degradation opens pathways for the uptake and migration of moisture due to the “vapor drive” caused by the temperature difference through the thickness of the foam insulation system. Cracks permit incursion of moisture and humid air, which will form ice and greatly increase the surface area for heat transfer. The sealing materials used tend to further trap the water inside the system; the moisture intrusion can come in one small spot even though the rest of the system remains sealed.
Other insulation systems useful in cryogenic applications include evacuated annular spaces having bulk-filled materials, e.g., glass fiber, silica aerogel or composites. As with MLI, these systems require high vacuum levels of around 1×10−3 torr.
Cryogenic insulation system performance is often reported for large temperature differences in terms of an effective thermal conductivity, or k value (ke). Boundary temperatures of 77K (liquid nitrogen) and 293K (room temperature) are common. Unless otherwise noted, k values discussed herein apply generally to these boundary conditions, as described by industry standard ASTM C1774.
MLI systems can produce ke of below 0.1 mW/m-K (or R-value of approximately 1440) when properly operating at cold vacuum pressure (CVP) below about 1×10−4 torr. For bulk-filled insulation systems operating at CVP below about 1×10−3 torr, ke of about 2 mW/m-K (R-value of approximately 72) may be typical. Foam and similar materials at ambient pressures typically may produce ke of about 30 mW/m-K (R-value of approximately 4.8). It should be noted that a ke of 1 mW/m-K is equivalent to an R-value of 144.2. R-value is a standard industry unit of thermal resistance for comparing insulating values of different materials. It is a measure of a material's resistance to heat flow in units of ° F.-hr-ft2/BTU-in. All values given as typical above represent one inch thickness of insulation of the type described, at ambient air pressure conditions with boundary temperatures of approximately 300K and 77K, as defined by industry technical standards including ASTM C168 and ASTM C1774.
Insulation systems are known which have low thermal conductivities at high vacuum conditions, but their performance depends on the level of vacuum and degrades precipitously as pressure is increased above 1×10−3 torr. Other insulation systems are capable of operating at ambient pressure, but do not exhibit sufficiently low thermal conductivity for most cryogenic applications and are difficult to protect against moisture and air intrusion or do not hold up well in the outdoor or ambient environment. Accordingly, there is a need in the art for systems of thermal insulation having reasonably low thermal conductivity that offers consistent, stable, long-term performance in the ambient environment (e.g., weather exposure).
Ambient air, or exposed, insulation systems for low-temperature, i.e., sub-ambient, applications are difficult to achieve because of moisture ingress and environmental degradation as well as thermal stress-cracking. All currently accepted methods are fraught with problems centered around moisture and sealing. Current technology for foams and blankets are practical for installation only on “clean” free, large area surfaces that are free from complications from supports, flanges, ports, valves, structures, etc. Such “clean” systems are rarely found, leading to severe performance and life-cycle maintenance problems due to the variety of complications, imperfections, or terminations throughout the system.
The conventional wisdom and most engineering training related to thermal performance provides that if a good thermal insulation capability is required then a suitably good thermal insulation material is all that is needed. This thinking is almost always faulty, especially for sub-ambient and cryogenic applications, because of the complexity of the thermophysical processes on the surfaces of the systems exposed to the ambient environment as well as the complexity of the mechanical elements to be insulated. It is the rendered thermal insulation system that delivers the performance needed, not the specification of an individual (component) material. For example, specification of the widely used cellular glass material (FOAMGLAS®) is only the starting point as a sophisticated system of mastics, sealants, expansion joints, face sheets, and binding hardware is also required as part of its rendered, field-installed system.